Traditionally, two types of legacy telecommunication networks have been developed. The first type is connection oriented and is used for the transport of narrow band voice traffic, typically carried in TDM frames. Such networks comprise for example synchronous or plesiochronous networks. The second type legacy network is connectionless in nature and is used for the transport of broad band packet or cell-based data traffic. There is currently a drive towards unified networks, which provide end to end transport for both voice and data services. However, as there is a well established voice network base, network operators are naturally reluctant to replace such legacy networks. This issue has been addressed by providing broad band (asynchronous) overlay networks which interface with the established TDM networks to provide a voice and data transport function. At the interface between the two networks, an interface function maps TDM frames into packets or ATM cells and vice-versa. ATM is of course just one example of a packet based network.
A particular problem with such an arrangement is the delay that builds up in the system when multiple packets or cells are scheduled for dispatch together (i.e. payloads that are waiting for transmission whilst other payloads are transmitted). Such delay can occur when packetising synchronous data, for example TDM traffic being packetized into fixed length voice packets such as ATM cell payloads, wherein multiple payloads from multiple data structures may be completed at substantially the same instant. The worst case example of such delay would be if every single data structure was scheduled to dispatch a cell at the same instant, which could occur naturally for example if all ATM VCCs were set up at the same time instant, such that at frame 1 the first octet of data arrives for each structure and forty six frames later (assuming the use of ATM Adaptation Layer 1 single channel adaptation) the last octet to complete the payload for each structure would also arrive. The scheduler then attempts to dispatch every cell instantaneously and a very large back-log is created which can take up to the super-frame period to eliminate (if the transmission bandwidth is equal to the data generation rate)—at which point it builds up again. In this example the super-frame is defined as the forty seven frame interval that it takes to assemble a completed packet.
The process is further complicated by the fact that, although the data samples are generated regularly there may be slight non-synchronization between the transmit and receive function. This results in timing slip which requires appropriate measures for rectification.